Answer: 2 x 2 x 2 x 2 x 2 x 5 x 5
Explanation:
We start with the string 2 x 2 x 2 x 2 x 5 x 5
Then we tack on a "2 x" up front, or at the very left, so that we double the result. This is because the jump from 400 to 800 is "times 2".
So we go from 2 x 2 x 2 x 2 x 5 x 5 to 2 x 2 x 2 x 2 x 2 x 5 x 5
We can condense this into